Apparatus for converting advancing sound waves into stationary waves



Dec. 23, T941. 3, L ovs 2,267,353

APPARATUS FOR CONVERTING ADVANCING SOUND WAVES INTO STATIONARY WAVES Filed July 31, 1940 Patented Dec. 23, 1941 ass-1.35s

APPARATUS FOR CONVERTING ADVANCING SOUND WAVES INTO STATIONARY WAVES Georges Lakhonky, Paris, fiance Application July 31, 1940, Serial No. 345,890

' In France August 11, 1939 4 claim. (on. lei-0.5)

The present invention relates to acoustic means which will be described hereinafter and which have the essential eflect of converting advancing sound waves into stationary waves.

Whenasoundisemittedinahallorina studio, the reproduction of said sound by the microphone is more faithful than in the open air because the vibrations of the air in a limited medium contain more harmonics than when the sound is emitted outside, so that the timbre formed by the whole of said harmonics is of a richer and more accurate quality. In other words, the obtaining of a larger number of harmonics, which is the condition for faithful reproduction, is the direct result of the fact that in a restricted space, such as that of a hall, the sound t'ibrations emitted in the form of stationary waves retain to a a certain extent their character of stationary waves.

On the contrary, a sound which is emitted.in the open air fades away more rapidly, so that a microphone that is placed in the open air transmits nasal and imperfect sounds since the same are deprived of a large part of theirharmonics on which the timbr depends.

On the other'hand, when an orchestra or a chorus is giving an audition in the vicinity of a microphone, it is possible at the receiving stations to obtain a proper reproduction of the instruments or of the singers that are located a short distance, for example a few metres from the microphone. n the contrary, those located at a greater distance, from thirty to forty metres for example, can onlybe heard faintly or with a considerable distortion of the sounds transmitted by the microphone. With the ear, on the contrary, musical sounds or the voice are readily heard at much greater distances which may attain more than one hundred metres.

In the first case the waves are advancing and fade away very quickly with the distance, whereas in the second case stationary waves are formed in the mass of air that occupies the ear passage in front of the drum, said mass of airin this case vibrating integrally, hence the extreme sensitivity of the normal ear at long distances.

In order to construct apparatus which is as nearly as possible like the structure of the ear, I arrang the microphone adjacent a chamber which is closed on one of its faces by a plate provided with orifices into which the sound waves penetrate and are then converted inside the chamber into stationary vibrations which then comes out through the same orifices and are received by the microphone. The resulting efiect is in this case identical with that which occurs in the reception of sounds by the ear and it has been found that a pure audition and a sensitivity of the microphone are systematically obtained and are maintained even when the sound is emitted at a comparatively long distance from the microphone.

The present invention gives quite remarkable results for the improvement of the sounds emitted by the loud-speakers of motion picture theatres, or the like.

In fact, when the soundsproduced by an orchestra for example, issue from a loud-speaker horn of small siz of about 25 centimetres diameter, the vibrating mass of air which is produced by said horn is much smaller than that produced by the orchestra which is grouped in a large hall, so that the volume of sound air produced by the'loud-speaker is comparatively small with respect to the large motion picture hall, the volume of air of which is considerable. This small mass of sound air is therefore, so to speak, neutralized by the large mass of surrounding air of the hall, hence the disappearance of the higher .harmonics, which causes distortion.

But if the device which is the object of the present invention is placed in front of said loudspeakers, the sound vibrations first of all enter the chamber of said device, which is limited by the perforated plate and in which chamber the stationary waves are formed. Thes latter then issue through the holes of said plate and are again reflected toward the loud-speaker, so that with the forward and return movement of these reflected waves, all the harmonics com out integrally. l

The objects and features of the invention as hereinabove outlined will be more apparent from a consideration of the annexed drawing and the following detailed description wherein an exemplary embodiment of the invention is disclosed.

In the accompanying drawing:

Fig. 1 shows an elevation in part section of a. device added to a microphone and provided with the features of th invention;

Fig. 2 shows a likewise elevational view with a certain part in section of an arrangement for improving the sound of the motion picture hall loud-speaker.

Referring to Fig. l, the housing E, which in the example shown is of hemispherical shape but which might equally well be a paraboloid, or an ellipsoid, is closed on the side. facing the microphone by a plate P provided with perforations p.

The housing E and the plate P may be formed of metal or any other suitable material such as plastic or rubber.

Said perforated plate may be extended by a screen P which may be perforated and which serves to reflect the advancing sound waves arriving behind the microphone from the sources of sound and send them towards the microphone.

The casing I of the microphone (not shown) is fixed in a housing L by means of a ring 2 made of resilient material, in particular of rubber. The

edge of the housing L carries a cross-piece 3 which is mounted thereon by means of bolts 4. The cross-piece carries a threaded rod 5. A nut 6, secured to the housing E, is adapted to screw on the rod 5 in order to adjust the distance between the housing E and the casing I of the microphone.

When the sound waves arrive in front of the microphone within the casing I, they are directlyconverted into stationary waves by reflection in the closed housing E and are sent back towards the microphone. If the sound waves arrive from the rear of the microphone, or from the sides, the perforated plate P forms a reflector and again sends such waves back towards the microphone. It is therefore possible with this device to obtain faithful auditions of sounds emitted "from a comparatively long distance from the microphone,- and it is no longer necessary, even in the case of the audition of a large orchestra or of the noises-of a crowd, to arrange a plurality of microphones which, owing to the different distances, do not always operate in synchronism, which is an additional cause of distortion of the voice or of the musical sounds.

The device according to the invention is provided with adjusting means which are preferably formed by the central screw 5 which enables the housing E in which the stationary waves are formed to be mounted more or less remote from the microphone and to accurately adjust the reflection and the intensity of the stationary waves that are sent back towards the microphone.

Such a device can be used, with a few adaptations of mounting, to receive the waves issuing from a loud-speaker and which are thus converted into stationary waves. The perforated plate P in this case forms a reflector and does not allow the distorted waves to spread into an audition hall. The housing provided with the perforated plate enables the advancing waves to be converted into stationary waves which propagate in the hall with all their harmonics and retain a great purity.

'If a loud-speaker H is located at a fairly short distance behind a projection screen F of large size and a housing E1 is placed between the loud-speaked and the projection screen, the whole of the mass of air between the projection screen and a baffle B that supports said loud-speaker starts vibrating throughout with the same density of stationary waves as the vibrating air issuing from the housing E1, so that such a mass of air.

which is considerably increased and amplified and is vibrating with stationary waves, makes its effect felt over a considerable area, for examplea few square meters, which is amply sufiicient to cause the air of the large hall to participate in these stationary wave vibrations and this is what enables a sound reproduction of remarkable purity to be obtained.

A still better result is obtained when the housing E1 is fixed on a board, or a screen R, the area of which is several times larger than the opening of the loud-speaker H. The board R and the housing E1 may be suspended in a cable C which passes over pulleys q so that the board R may be raisedor lowered.

In front of the opening of the loud-speaker H a reflection or a forward and return movement occurs which, to a certain extent, converts these vibrations into stationary waves. These improved waves propagate throughout the whole mass of air between the bafile and the projection screen and in turn send into the hall sounds of superior quality.

While I have-illustrated and described the preferred forms of construction for carrying my invention into effect, these are capable of variation and modification, without departing from the spirit of the invention. I therefore do not wish to be limited to the precise details of construction set forth, but desire to avail myself of such variations and modifications as come within the scope of the appended caims.

I claim:

1. Apparatus for improving the tone quality of Y microphones comprising, a microphone casing, a housing, yleldable means supporting the microphone casing in said housing, a second housing, a perforated plate facing said microphone casing and covering the second housing to provide an air chamber within the second housing, and means carried by 'the'first housing supporting the second housing and said perforated plate adjacent said microphone casing whereby advancing sound waves impinging on the front of the microphone casing are reflected through the perforations of said plate into said'c'hamber and therein converted into stationary waves and the sound waves impinging on the rear or the sides of the microphone casing are reflected by said plate into the microphone casing.

2. Apparatus for converting advancing sound waves into stationary sound waves adjacent a microphone casing comprising, a housing, yieldable means supporting the microphone casing in said housing, a second housing, a perforated plate facing said microphone casing and covering the second housing to provide an air chamber therein, and means carried by the first housing adjustably supporting the second housing adjacent said microphone casing whereby the distance between said plate and said microphone casing may be adjusted.

3. Apparatus for converting advancing sound waves into stationary sound waves adjacent a microphone casing comprising, a hemisphericalshapedhousing, yieldabl'e means supporting the microphone casing in a wall of said housing, a second hemispherical-shaped housing facing the first housing, a perforated plate facing the microphone casing and covering the second housing to provide an air chamber within the second housing, a threaded rod supported by the first housing, threaded means carried by the second housing engaging the threads of said rod whereby the distance between the first and second housings 

